Diamine curing agents are often favored as curatives for epoxy resins used in structural composites. J. N. Owens, "Dielectric Curing of Epoxy Resins: Measurements of Dielectric Properties at Radio Frequencies", G. M. Research Report PO-844, Jun. 4, 1987, the diamine curatives yield resin mixtures with low dielectric loss factors during the cure. On the other hand, processes are presently being employed utilizing a dielectric curing process for the fabrication of structural composites. The use of dielectric curing enables the rapid curing of thick cross sections and thereby reduces cycle times. With conventional heating, the thick cross sections are heated by conduction from the outside surfaces, whereas with dielectric heating, heat is generated within the entire volume of the part. The volumetric heating enables faster heating rates and shorter cure times. However, the low loss factors of the diamine cured epoxy resins make dielectric heating difficult. To take advantage of the dielectric heating process, a resin with high loss factor is desirable.
Carbon blacks are known to increase the dielectric loss factor of epoxy resins, however, most carbon blacks cause a significant increase in resin viscosity, except at very low concentrations. Carbon blacks upwardly shift the entire loss factor curve of the resin but the shape of the curve is unaffected. At very low carbon blacks concentration, the effectiveness of increasing the dielectric loss factor of epoxy resins by the mere addition of carbon blacks is questionable.
In a previously filed patent application by this inventor now U.S. Pat. No. 5,057,554, a method of using a chemically inert salt soluble in a resin for increasing the dielectric loss factor of the composition without significantly increasing the viscosity of the composition was disclosed. Preferably, quaternary ammonium salts are used which have been unexpectedly found to increase the dielectric loss factor of epoxy resins. However, the salts are chemically inert in the resin.
The U.S. Pat. No. 4,554,341 to Allen, issued Nov. 19, 1985, discloses the addition of tetrabutylammonium bromide and similar salts for use as curing accelerator compound in an epoxy composition. The U.S. Pat. Nos. 4,358,577 to McCrary, issued Nov. 9, 1982, 4,284,753 to Hewitt, Jr., issued Aug. 18, 1981, and 4,725,652 to Bertram et al, issued Feb. 16, 1988, all disclose tetrabutylammonium salts for use as curing accelerators. In each of the aforementioned patents, the quaternary ammonium salt additives act as chemical accelerators to the curing reactions. The additives are not inert but rather are chemically active participants in the curing reaction. The quaternary ammonium salts are not inert in the systems and there is absolutely no recognition of the quaternary ammonium salts having the ability to affect dielectric heating. Moreover, none of the aforementioned patents utilize or contemplate the dielectric heating of the chemical compositions per se.
It is, therefore, an object of the present invention to provide a method of increasing the dielectric loss factor of epoxy resins by using an additive system that is more effective than the addition of either carbon blacks or quaternary ammonium salts alone.
It is another object of the present invention to provide a method of increasing the dielectric loss factor of epoxy resins by using an additive system that does not increase the viscosity of the epoxy resins to a significant degree so as to impair their processing characteristics.